1. Field of the Invention
The present invention relates to an information storing apparatus for inputs/outputs information by rotating a disk-like recording medium, e.g., a CD-ROM, at a constant linear speed and, more particularly, to an information storing apparatus in which the driving system for the recording disk is improved to achieve cost reduction, power savings, low noise, and the like.
2. Description of the Prior Art
In an information storing apparatus which inputs/outputs information by rotating a recording disk, e.g., a CD-ROM, at a constant linear speed with respect to a pickup, the recording disk is driven at different rotation speeds in accordance with the positions of the pickup. Accordingly, in such an information storing apparatus, in the ordinary read mode, the rotation speed of the recording disk must be gradually and accurately changed, while in the seek operation, the speed must be changed quickly in order to shorten a seek time, requiring a very large accelerating/decelerating torque when compared to the torque required in the read mode. For this reason, in general, a highly controllable and highly reliable DC brushless motor is used as a driving motor for driving the recording disk.
However, the DC brushless motor is expensive as it needs position detectors, e.g., Hall elements, and is controlled mainly by analog control as it requires speed feedback control, leading to a large circuit configuration. For these reasons, the resultant system cannot sufficiently meet the recent demand for cost reduction.
With recent advances in the digital electronics technology, a technique has been established that can freely control the rotation speed of a two-phase claw-pole stepping motor as a synchronous motor in accordance with the microstep driving scheme. According to this method, a digital control circuit can be constituted without adding any position detector or a speed detector in the motor. As a result, the motor can be rotated at high rotary precision, and the cost of the entire system can be reduced.
When, however, a stepping motor is used as a synchronous motor, unlike in a DC motor, the motor current does not increase or decrease in proportion to the increase or decrease of the load. Therefore, this motor cannot cope with a wide range of torque fluctuations of the load, causing an out-of-step phenomenon in the worst case. As a countermeasure against this problem, a current corresponding to the maximum load is constantly supplied. In this case, however, a large current flows even under a light load in, e.g., the ordinary read mode, which is uneconomical. In addition, since a large current flows, the vibrations, noise, heat, and the like of the motor are large.